«Report prepared by Davey Jones and Bridget Emmett August, 2013 This report was co-funded through the Welsh Government Land Use and Climate Change ...»
Actions to minimise loss of current soil carbon and
enhance soil carbon sinks in Wales
Report prepared by Davey Jones and Bridget Emmett
This report was co-funded through the Welsh Government Land Use and Climate Change Committee and the
Seren programme. Seren is part funded by the European Regional Development Fund. Cardiff University,
Bangor University and the Centre for Ecology and Hydrology wish to acknowledge the support provided to the
project by the Welsh European Union Funding Office (WEFO).
1. Introduction Welsh soils hold a large reserve of organic carbon, however, this store is vulnerable to loss through land use change and anthropogenic perturbation (e.g. climate change). If soil carbon is lost it causes the release of greenhouse gases (e.g. CO 2, CH4, N2O) and has negative effects on other ecosystem services such as food security, biodiversity, and water storage. Similarly, there is potential to store more carbon in the soils of Wales. It is therefore vital that we preserve the nation’s terrestrial carbon store. Similarly, there is potential to increase carbon storage in soil through changes in agricultural management and the use of waste materials.
In this report we use the best available evidence to estimate the potential effect that changes in land use and agricultural management can play in reducing greenhouse gas (GHG) emissions and enhancing carbon storage in Welsh soils. GHG emissions from animals are not directly considered here. We considered a wide range of mitigation measures the lifespan of which ranged from 1-50 years and considered evidence for the three major GHGs; carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). Most evidence from the literature was for changes in CO 2 or the surrogate, soil carbon storage. Fewer studies were available for N 2O and very few studies for CH4. Literature values were collated for all intervention measures and the midpoint of the range reported applied to landcover/soil type combinations present in Wales, except where Welsh studies indicated different values should be used. We evaluated the potential for different land use change strategies to mitigate against GHG over a 50 year timescale (i.e. 2009-2050) and with different levels of farmer adoption. It is assumed all activities are undertaken in Year 1 and the outcomes for soil carbon and GHG emissions are reported over the subsequent 50 year period.
It is important to recognise that although increasing soil carbon can have substantial benefits to the structure and productivity of soils, there is poor evidence about the linkage between many land management activities and changes in soil carbon and net GHG emissions – taking account of CO2, CH4 and N2O. Photosynthetic processes which fix CO2 in the soil and biomass on a permanent basis are key to reducing the concentrations of the gases in the atmosphere and protection of soil carbon stocks already accumulated should remain a priority.
It is clear that the potential to reduce GHG emissions in soils and biomass has some limitations. There is a limit to soil carbon storage which is possible after any land use or management change because a new equilibrium is reached, so called ‘saturation’.
Our work indicated the temporal dynamics of the combined activities reached a maximal in the first 30 years but reaches close to saturation after 50 years. To put this into perspective, we calculated the sum of these GHG reduction measures and plotted them against the 2005 baseline agricultural emission figures. Clearly, if these intervention measures are adopted they have the potential to make a difference. If we are looking for a 3% reduction in emissions each year relative to the 2005 baseline then we would be expecting a 30% reduction in 10 years. This is clearly possible. After this point other mitigation measures would need to be revised if the targets are to be met. There also has to be safeguards in place to ensure the permanence of the reduction measures and that benefits in one location are not negated by enhanced emissions elsewhere – so called ‘displacement’. We also emphasise that the potential trade-offs and co-benefits of land management options 2 for GHG emissions reductions and other requirements from the land need to be objectively quantified together with full life cycle assessment to ensure the desired outcome on GHG across sectors would be achieved.
2. Greenhouse gas flows and carbon stocks For Wales in 2007, land use change to forestry and grassland sequestered 2084 kt carbon dioxide equivalent (CO2 e), and conversion of land to cropland and settlements led to emissions of 1875 kt CO2 e. In total, land use change led to a net sink of 199 kt CO2 e. On present trends, the position by 2020 is that the emission rate will be greater than the sink rate because current Welsh forests are becoming mature with reducing rates of growth and carbon uptake. The major components of these sinks and sources are shown in Table 1.
Table 1. Emissions and removals of GHG by Land Use, Land Use Change and Forestry (LULUCF) in 2007, year average and the general trend (Thomson, 2008).
Values are in CO2 equivalents (CO2 e).
These quantities are based on changes in biomass and soil carbon levels, and associated fluxes of GHGs. Agricultural land use changes are a significant driver of
these changes. Mitigation opportunities exist in four main areas:
1. Minimising emissions by conserving soil carbon stocks in organic soils e.g.
2. Enhancing net GHG sequestration in organic and mineral soils by improved grassland, and woodland and wetland management.
3. Enhance carbon in plant/tree biomass.
4. Minimise N2O losses from all soils through better management of nitrogen fertiliser use.
An important factor determining sinks and sources is the high soil organic content of Welsh soils, mainly associated with permanent grassland and the uplands. The most accurate estimate of the carbon stock of Welsh soils is obtained by aggregating comparable data derived from Bradley et al. (2005) and Smith et al. (2007). While Smith et al. (2007) provides a more complete estimate of total stock, the data of 3 Bradley et al. (2005) are currently used to calculate emissions estimates in Greenhouse Gas Inventories for Wales.
Based on the work of Smith et al. (2007) and Bradley et al. (2005), the Welsh soil carbon stock is estimated to be 409 Mt carbon (1500 CO2 e). However, it must be recognised that estimates of soil carbon reserves are heavily reliant on the quality of soil maps (degree of ground truthing, map scale, classification type) and on algorithms describing carbon density in soil (Frogbrook et al., 2009). Consequently, estimates of national soil carbon storage from different mapping approaches gives a range of 340-530 Mt carbon (mean 436 ± 27 estimated from 7 different datasets/national soil maps equivalent to 1600 ± 100 Mt CO2 e; Ibn Malik, 2006).
Approximately half of the total soil carbon stock is located within an area of 492721 ha or 23.4% of the land surface of Wales, predominantly in upland areas and / or areas of permanent grassland. The remaining 76.6% of Wales is covered primarily by mineral soils with low carbon content (Figure 1).
Figure 1. Distribution of soil carbon in Wales The left hand panel represents the amount of carbon stored from a depth of 0-15 cm and the right hand panel from a depth of 0-100 cm (Ibn Malik, 2006).
This report reviews mitigation options in terms of three main soil types, namely;
mineral soils, organo-mineral soils, and organic (peat) soils. Soil type is paramount when considering impacts of agricultural operations and land use change, as different soils react differently to the same operation. So a certain operation undertaken on organic soils may reduce emissions while the same operation on mineral soils results in increased emissions. The land cover types were defined using the CEH Land Cover Map 2000 and mapped onto soil types using the NATMAP vector soils data. A summary of the classes and soil type distribution within each land use type is provided in Table 2.
In this report, it was decided to use the Countryside Survey (Carey et al., 2008) as the basis of scenarios for land use change, because the GHG inventory is currently based on the map, and it is updated at regular intervals during the Countryside 4 Surveys. Methods of improving the LULUCF inventory are being assessed, with particular concern to improve input statistics to track land use change with time – for example, using agriculture statistics data, National Forest Inventory data and Natural Resources Wales Phases1 and 2 data.
(i) Based on CEH Land Cover Map 2000 25 m data and NATMAP vector soils data (i.e. essentially land use cover versus soil type based on 1998 data) (ii) No LCM class 43 (arable non-rotational) mapped in Wales (iii) No classification for man-made soils into mineral, organic, peat (iv) Results based only on areas where data for both land cover & soils are available
63. Key emission and carbon loss processes
Carbon is largely lost from soil as carbon dioxide (CO2) as a result of the natural breakdown of soil organic matter by soil microorganisms (Paul and Clark, 1996). The rate of soil carbon loss is maximal in warm, relatively moist and aerobic conditions.
This process is also exacerbated by physical disturbance which breaks up soil aggregates, enhancing oxygenation and allows microbial access to physically protected carbon (e.g. ploughing). Methane (CH4) is produced when organic materials decompose in oxygen-deprived anaerobic conditions, such as permanently waterlogged soils. Nitrous oxide (N2O) is also generated when soil microorganisms run out of oxygen (e.g. in very wet or compacted soils) and occurs when there is lots of available nitrate (where available nitrogen exceeds plant requirements). It is also exacerbated after addition of nitrogen rich organic wastes to wet soils. N2O is a much more potent greenhouse gas than CO2 and CH4, Therefore, even though loss rates can be small, the resulting climate change effect can be very large.
The exchanges of carbon between the land and atmosphere is dominated by the emission and plant fixation of inorganic carbon (ca. 43.3 Mt CO2 e/y as Net Primary Productivity (NPP)). Emissions from vegetation include both CO2 through respiration but also non-methane volatile organic carbon (NMVOC) back to the atmosphere which is a precursor of ozone which contribute 3-7% of the greenhouse effect.
Although there is uncertainty in the figures, particularly for grazed grasslands, current estimates for NMVOC loss rates to the atmosphere range from 0.18 to 1.8 Mt CO2 e/y which equates to a loss of 0.1-1.0% of the total carbon held in vegetation each year representing ca. 0.5-4% of the net carbon fixed by plants in photosynthesis (Guenther, 2002). As forests release more NMVOC’s than grassland, it is likely that total NMVOC emissions from Wales are at the lower end of the emission range. The climate change impact of NMVOCs is discussed in Stewart et al. (2003) and Laothawornkitkul et al. (2009).
In reviewing actions to reduce emissions, estimates of soil carbon loss have been based on assuming that all loss is as carbon dioxide and methane emitted to the atmosphere. However, carbon can also be lost from soil as dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC) either from surface erosion, runoff or leaching (Worrall et al., 2007). Monitoring of water quality in Wales has shown that there has been a significant increase in dissolved and particulate organic carbon over the past 20 years, attributed to several factors including reduction of acid deposition and climate change (Evans et al., 2005).
Estimates for Wales suggest that the annual loss of carbon to freshwaters is 1.5 Mt CO2 e with a level of uncertainty ranging from 0.84 to 2.90 Mt CO2 e. Of this loss, approximately 40% is as CO2 (due to CO2 degassing and in-stream breakdown of POC and DOC).
Increasing the soil carbon content can only occur either by increasing carbon input, decreasing carbon output or by a combination of the two through improved land management. Agricultural management systems and forestry operations can strongly influence soil processes such as carbon sequestration and erosion.
Examples include drainage of and cultivation of waterlogged organic soils, leading to aeration, increased microbial decay and an associated increase in CO2 emissions, but decreases in N2O emissions. Intensive arable use of mineral soils can enhance 7 N2O emissions due to the increased rate of de-nitrification associated with excess fertilizer applications, yet it is known that nitrogen is an important driver for fixing more carbon in soils. This emphasises the need to look at all GHG fluxes, and not focus solely on soil carbon. A major gap in our information is an accurate account for all GHG fluxes to and from Welsh soils.
For this sector it is important to recognise that there are significant feedback mechanisms, whereby climate change can exacerbate emissions from soils. It has been reported that topsoils in England and Wales have lost significant quantities of soil carbon over the last 25 years, possibly due to the effects of climate change (Bellamy et al., 2005). This landmark study by Bellamy et al. (2005), however, has been intensively criticized (Smith et al., 2007; Potts et al., 2009; Stutter et al., 2009;
Reynolds et al., 2013) highlighting the difficulties in assessing changes in soil carbon storage over short time scales (25 years; Prechtel et al., 2009). A second national monitoring programme called Countryside Survey has recently reported results from a similar time period indicating no evidence of topsoil carbon concentration or stock decrease at GB or individual country level (Carey et al., 2008; Reynolds et al., 2013).